Water purification typically produces a first effluent of relatively “clean water” and a second effluent of “waste water” which includes unwanted contaminants.
For example, the softening of hard water by the removal of calcium and magnesium is required for both industrial and household use. Known water softening processes proceed either by way of ion-exchange, membrane softening or precipitation. In the ion-exchange processes, the calcium and magnesium ions are exchanged for sodium ions. Regeneration of the ion-exchange resin is achieved with a large excess of sodium chloride. This process creates a regeneration effluent being a relatively concentrated aqueous solution of sodium, calcium and magnesium chlorides. Consequently, by this method, considerable amounts of sodium, calcium and magnesium salts in solution must be disposed of.
Alternatively, it is possible to use weak acid resins which exchange hydrogen for calcium and magnesium ions, and to regenerate the spent resins with a mineral acid. While this method creates less waste water, it is more expensive and yields relatively acidic soft water which is corrosive.
Membrane softening concentrates the calcium, magnesium salts and salts of other divalent ions to produce waste waters which require costly disposal.
The precipitation process has traditionally been carried out by the “lime soda” process in which lime is added to hard water to convert water soluble calcium bicarbonate into water insoluble calcium carbonate. This results in waste water which is difficult to filter and requires use of cumbersome equipment.
It would be desirable if salty waters, e.g., salty irrigation drainage, could be processed to provide a recycle stream of clean water and a waste stream containing salts that can be processed by electrolysis for producing chlorine products and byproducts.